On Saturday, May 18th, I was a passenger in a car just on the outskirts of Tucson, AZ about 15 minutes past sunset when the driver noticed a large metallic object in the sky out the driver side window. It appeared stationary and so I thought maybe it was a large airplane heading towards us. I didn't see any lights, but it was high enough up that it was still illuminated by the sun. I wasn't able to take a picture, but it showed up on FR24 as a "ball":

I couldn't find any news articles that explained the "ball". But, I came across an article, "World View Touts Success, Growth" about a company in Tucson that launches stratospheric balloons for commercial purposes. The article has a picture of one of their balloons. If this is what I saw on Saturday, I'm surprised at how much bigger it seemed to be up in the sky at 67,100 ft (according to FR24) than it appears in the photo in the article.

Interesting. The video you posted says that what they previously launched back in 2014 was 1/10th the size of what they want to launch for passenger flights. So maybe what I saw was a prototype somewhere between the size shown in that photo and what their final goal is. I don't have a good sense of how big an object at 67,000 ft should have appeared at the elevation I was at (I think ~2,000 ft). But it just seems to me that the balloon in the photo would have been barely visible, if at all, and what I saw the other evening was pretty easy to spot the instant I looked out the window.

Flightradar24 shows that The ball/balloon is still aloft - currently over the Grand Canyon at 62,500 ft.

Interesting. The video you posted says that what they previously launched back in 2014 was 1/10th the size of what they want to launch for passenger flights. So maybe what I saw was a prototype somewhere between the size shown in that photo and what their final goal is. I don't have a good sense of how big an object at 67,000 ft should have appeared at the elevation I was at (I think ~2,000 ft). But it just seems to me that the balloon in the photo would have been barely visible, if at all, and what I saw the other evening was pretty easy to spot the instant I looked out the window.

Flightradar24 shows that The ball/balloon is still aloft - currently over the Grand Canyon at 62,500 ft.

Click to expand...

These balloons tend to expand as the air pressure drops. They are under-inflated at launch to have extra material for the expansion.